Method for charging substrate to a potential

ABSTRACT

A surface of an insulating substrate is charged to a target potential. In one embodiment, the surface is flooded with a higher-energy electron beam such that the electron yield is greater than one. Subsequently, the surface is flooded with a lower-energy electron beam such that the electron yield is less than one. In another embodiment, the substrate is provided with the surface in a state at an approximate initial potential above the target potential. The surface is then flooded with charged particle such that the charge yield of scattered particles is less than one, such that a steady state is reached at which the target potential is achieved. Another embodiment pertains to an apparatus for charging a surface of an insulating substrate to a target potential.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional patent application of parentpatent application Ser. No. 10/942,184, filed Sep. 16, 2004, entitled“Method for Charging Substrate to a Potential,” by inventors Kirk J.Bertsche and Mark McCord. The disclosure of the aforementioned parentpatent application is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatus and methods for chargingspecimens to a potential.

2. Description of the Background Art

It is often desired to charge a non-conducting substrate to a known oruniform electrostatic potential. For example, this is often desired in acharged-particle instrument, such as an electron microscope, ionimplanter, or other such instrument. One common application is thecharging (or discharging) of a semiconductor wafer, especially with aninsulating layer thereon. For example, the insulating layer may besilicon dioxide or another insulating material.

A common method for such charging utilizes electron flooding. Theconventional technique for such electron flooding is now discussed inrelation to FIGS. 1A and 1B. FIG. 1A is a flow chart depicting a method100 of conventional flooding. FIG. 1B is a schematic diagram depicting aconfiguration for the conventional flooding in cross-section.

In the conventional flooding technique, the flood system is configured(102) to generate an electron flood beam 152 with a relatively highlanding energy (typically, a few hundred electron volts) such that theelectron yield is greater than one (η>1). The substrate 154 is passed(or is positioned) (104) under the electron flood gun 156, while anelectrostatic field is simultaneously applied (106) above the substrate154. The flood gun may include, among other components, a cathode 157,an anode 158, and a lower bias electrode 159. The electrostatic fieldmay be applied, for example, using the lower bias electrode 159 aroundthe flood gun 156 (and perhaps a grid in front of the flood gun).Secondary electrons 160 are emitted (108) from the surface 153 of thesubstrate 154 with very low energy (typically a few eV). These lowenergy secondary electrons 160 move under the influence of theelectrostatic field, charging (110) the surface 153 until theelectrostatic field is essentially neutralized. This results in asurface potential approximately equal to the potential of the lower biaselectrode 159 of the flood gun 156. However, the potentials are notexactly equal because of space charge effects. When the surface chargereaches (112) a steady state, a secondary electron current 160 (equal tothe incident flood beam current) continues to leave (114) the surface153 of the substrate 154 and drift across the gap towards the lower biaselectrode 158. These secondary electrons create (116) a space chargesituation with a self-consistent field that drives the secondaryelectrons across the gap from the surface 153 to the flood gun biaselectrode 158 and causes (118) a voltage depression at the surface ofthe substrate 154.

The following is a typical example of configuring (102) the system forconventional flooding of a semiconductor wafer with a layer of silicondioxide on its surface. The configuration (102) may be performed using acontroller 151 that is configured to control the various voltagesapplied in the system, such as the voltages on the electrodes in theflood gun 156 and the voltage applied to the substrate 154. The cathode157 may be at a potential of negative three hundred volts (−300 V), andthe anode 158 of the flood gun may be at the ground potential of zerovolts (0 V), such that the electron flood beam has an energy of threehundred electron volts (300 eV). The voltage bias applied to the wafersubstrate 154 (the wafer bias) may also be at electrical ground (0 V).Assuming that the potential at the wafer surface is not too far from thewafer bias, this would result in a landing energy of roughly threehundred electron volts (300 eV). If the substrate surface 153 is to becharged to approximately ten volts (10 V), then the lower bias electrode159 of the flood gun 156 may be set to ten volts (10 V).

Unfortunately, as mentioned above, the conventional flooding typicallycauses (118) an unwanted voltage depression at the surface of thesubstrate 154. In a one-dimensional approximation, Child's Law may beused to calculate an approximation of the voltage depression. Forexample, given a one centimeter (1 cm) gap and an incident beam currentdensity of one hundred microamperes per square centimeter (100 μA/cm²),the space charge will depress the substrate by roughly twelve volts (12V), according to a calculation using Child's Law.

In addition to the aforementioned voltage depression, variations incurrent density in the flood beam and at the beam edges will causevariations in substrate surface charge in the conventional floodingtechnique. Furthermore, there may be additional substrate potentialerrors due to voltage drops at the flood gun electrode (caused byresistivity and/or work function effects at the electrode). Moreover,there may be further substrate potential errors due to stray electricfields. In practice, these various voltage offsets give rise to chargevariations of a few or several volts across the surface of a substrate.This makes it difficult to flood a substrate to a known or uniformpotential with accuracy of better than a few volts.

SUMMARY

A surface of an insulating substrate is charged to a target potential.In one embodiment, the surface is flooded with a higher-energy electronbeam such that the secondary electron yield is greater than one.Subsequently, the surface is flooded with a lower-energy electron beamsuch that the secondary electron yield is less than one.

In another embodiment, the substrate is provided with the surface in astate at an approximate initial potential above the target potential.The surface is then flooded with charged particles such that the chargeyield of scattered particles is less than one, such that a steady stateis reached at which the target potential is achieved.

Another embodiment pertains to an apparatus for charging a surface of aninsulating substrate to a target potential. An electron flood gunincludes a cathode, an anode, and a lower electrode. A substrate holderhas a substrate bias voltage applied thereto. A controller controls theflood gun and the substrate bias voltage. The controller is configuredto flood the surface of the substrate with a higher-energy electron beamsuch that the electron yield is greater than one. The controller is alsoconfigured to subsequently flood the surface with a lower-energyelectron beam such that the electron yield is less than one.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a flow chart depicting a method of conventional flooding.

FIG. 1B is a schematic diagram depicting a configuration for theconventional flooding.

FIG. 2A is a flow chart depicting a method of mirror flooding inaccordance with an embodiment of the invention.

FIG. 2B is a schematic diagram depicting a configuration for mirrorflooding in accordance with an embodiment of the invention.

FIG. 3 is a flow chart depicting a sequential method of charging asubstrate to a target potential in accordance with an embodiment of theinvention.

FIG. 4 is a flow chart depicting a multiplexed method of charging asubstrate to a target potential in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION

As discussed above, it is often desired to charge a non-conductingsubstrate (such as, for example, a semiconductor wafer with an insulatorlayer thereon) to a known or uniform electrostatic potential. Forexample, this is often desired in a charged-particle instrument, such asan electron microscope, ion implanter, or other such instrument. Toaccomplish such substrate charging, the present application disclosesoperating an electron flood gun in a very different way than theconventional technique discussed above. Advantageously, the techniquedisclosed in the present application may essentially eliminate orsubstantially reduce voltage offsets due to the above-discussed effectsof space charge, flood gun electrode potential, and/or stray electricfields.

One embodiment of the present invention relates to a mirror floodingtechnique that is quite different from the conventional floodingtechnique. The mirror flooding technique is now discussed in relation toFIGS. 2A and 2B. FIG. 2A is a flow chart depicting a method 200 ofmirror flooding in accordance with an embodiment of the invention. FIG.2B is a schematic diagram depicting a configuration for mirror floodingin cross-section in accordance with an embodiment of the invention.

In the mirror flooding technique, the electron flood beam 152 isconfigured (202) with a relatively low landing energy such that theelectron yield is less than one (η<1). This is opposite to theconventional technique that purposely uses a relatively high landingenergy so as to ensure an electron yield greater than one.

The substrate 154 is passed (or is positioned) (204) under the electronflood gun 156. In this charging mode, the surface 153 of the substrate154 will charge (206) negatively (due to absorption of floodingelectrons) until the surface 153 reaches the potential of the cathode157 of the flood gun 156 (rather than to the approximate potential ofthe lower bias electrode 159 of the flood gun 156 in the conventionaltechnique).

When a steady state is reached (208), the flood beam will not causerelease of secondary electrons, but rather the incident electrons 152from the flood beam reflect (mirror) (210) from the substrate surface153 to create a reflected electron beam 164. An extraction electrostaticfield configured above the substrate surface 153 using an extractionelectrode 166 re-accelerates and extracts (212) the mirrored electronsrelatively quickly, avoiding space charge effects. In this mirror mode,the substrate surface potential advantageously becomes relativelyinsensitive to potentials of the flood gun electrodes (other than thecathode 157).

The following is a specific example of configuring (202) the system formirror flooding of the substrate surface. The configuration (202) may beperformed using a controller 151 that is configured to control thevarious voltages applied in the system, such as the voltages on theelectrodes in the flood gun 156, the voltage on the extraction electrode166, and the voltage applied to the substrate 154. The cathode 157 maybe set at a potential of negative one hundred volts (−100 V), and theanode 158 of the flood gun may be set at the ground potential of zerovolts (0 V), such that the electron flood beam has an energy of onehundred electron volts (100 eV). If the wafer surface potential is to belowered from a higher voltage to ten volts (10 V), then the voltage biasapplied to the wafer substrate 154 (the wafer bias) may be set atnegative one hundred and ten volts (−110 V).

In order to assure that the electron yield is less than one during theabove-discussed mirror flooding 200, the substrate charge should firstbe either known or controlled so that the flooding beam 152 nowherestrikes the surface 153 with an energy above E₁, defined as the electronenergy at which the electron yield is equal to one.

If the surface charge state is unknown, then conventional flooding 100may be first applied to achieve an approximately known voltage. Thepresent application discloses two techniques combining the conventionaland mirror flooding. One technique is a sequential method 300 and isdiscussed below in relation to FIG. 3. Another technique is amultiplexed or “chopped” method 400 and is discussed below in relationto FIG. 4.

Alternatively, multi-step mirror mode flooding may be used. For example,there are situations where an upper bound on the highest possiblepositive potential on a substrate may be known. Such an upper bound maybe known based on the history of processing of the substrate or based ondielectric breakdown properties relating to the substrate. In that case,mirror mode flooding may be used first at a higher positive targetpotential, then at progressively lower target potentials, until thesubstrate is brought down closer to the target potential. For example, awafer may have been inspected under electron extracting conditions, suchthat the wafer becomes charged positively but in a nonuniform manner.For purposes of this example, assume that the wafer comprises an oxidelayer that can be charged to 100 volts before breakdown, so that it isknown that the upper bound to the potential of the substrate is +100volts. Further assume that the target potential for the wafer is zerovolts. In this case, the mirror mode flooding may be performed firstwith a target potential of +100 volts, followed by a mirror modeflooding at +67 volts, then at +33 volts, then at 0 volts.

FIG. 3 is a flow chart depicting the sequential method 300 of charging asubstrate to a target potential in accordance with an embodiment of theinvention. The method 300 of FIG. 3 may be applied to a substrate in anunknown charge state. The method 300 includes two stages.

The first stage involves a conventional flooding process (100) to bring(302) the substrate to an approximate (rough) charge state. Oneembodiment of a conventional flooding process (100) is described abovein relation to FIG. 1A and 1B.

The second stage involves a mirror flooding process (200) to achieve(304) a final (fine) charge state. One embodiment of a mirror floodingprocess (200) is described below in relation to FIG. 3A and 3B.

For example, the conventional flooding process (100) may be used tocharge the substrate to a few tens of volts above the final targetvoltage. Then, the mirror flooding process (200) may be used to providenegative charging to lower the potential to the final target voltage.

In a specific example, suppose that a semiconductor wafer with a layerof silicon dioxide thereon is desired to be charged to a potential often volts (10 V). First, the wafer may be conventional flooded (100).The conventional flooding may be accomplished, for example, using thefollowing configuration (102). The cathode 157 may be set at a potentialof negative three hundred volts (−300 V), and the anode 158 may be setat the ground potential of zero volts (0 V), such that the electronflood beam has an energy of three hundred electron volts (300 eV). Thevoltage bias applied to the wafer substrate 154 (the wafer bias) mayalso be at electrical ground (0 V). The beam energy is high enough suchthat the landing energy should result in an electron yield greater thanone. The lower bias electrode 159 of the flood gun 156 may be set tothirty volts (30 V) such that the wafer surface becomes charged toroughly thirty volts. This charging to 30 V is only rough because of theaforementioned voltage depression and other voltage variations. Second,the wafer may be mirror flooded (200). The mirror flooding may beaccomplished, for example using the following configuration (202). Thecathode 157 may be set at a potential of negative one hundred volts(−100 V), and the anode 158 of the flood gun may be set at the groundpotential of zero volts (0 V), such that the electron flood beam has anenergy of one hundred electron volts (100 eV). If the wafer surfacepotential is to be lowered from the roughly thirty volts (30 V) to tenvolts (10 V), then the voltage bias applied to the wafer substrate 154(the wafer bias) may be set at negative one hundred and ten volts (−110V). The final result is that the surface of the wafer is set to 10 Vwith improved accuracy over the conventional flooding technique.

FIG. 4 is a flow chart depicting a multiplexed or “chopped” method 400of charging a substrate to a target potential in accordance with anembodiment of the invention. In this method 400, the surface 153 of thesubstrate 154 is covered area by area, rather than all at once. Eacharea selected (401) may be exposed to one or more cycles, where eachcycle includes conventional flooding (100) to bring about (302) a roughcharge state, followed by mirror flooding (200) to achieve (304) a finalcharge state at the target potential.

While there are more areas to be covered (404), the next area isselected (401) and the flooding process (100/302/200/304) is moved tothat area. When all the areas are covered (404), then the process iscomplete. The movement to cover the desired surface area can either bein a step-wise fashion or a continuous motion (swathing).

In the above description, numerous specific details are given to providea thorough understanding of embodiments of the invention. However, theabove description of illustrated embodiments of the invention is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. One skilled in the relevant art will recognize that theinvention can be practiced without one or more of the specific details,or with other methods, components, etc. In other instances, well-knownstructures or operations are not shown or described in detail to avoidobscuring aspects of the invention. While specific embodiments of, andexamples for, the invention are described herein for illustrativepurposes, various equivalent modifications are possible within the scopeof the invention, as those skilled in the relevant art will recognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific embodimentsdisclosed in the specification and the claims. Rather, the scope of theinvention is to be determined by the following claims, which are to beconstrued in accordance with established doctrines of claiminterpretation.

1-7. (canceled)
 8. A method of charging a surface of an insulatingsubstrate to a target potential, the method comprising: flooding thesurface with a higher-energy electron beam such that an electron yieldis greater than one; subsequently flooding the surface with alower-energy electron beam such that the electron yield is less thanone; and covering the surface area-by-area, wherein each area is coveredfirst by the higher-energy beam flooding and second by the lower-energybeam flooding.
 9. The method of claim 8, a cycle of the higher-energybeam flooding and the lower-energy beam flooding is repeated for eacharea.
 10. The method of claim 8, wherein coverage of the surface isachieved in a step-wise fashion.
 11. The method of claim 8, whereincoverage of the surface is achieved using swathing. 12-14. (canceled)15. A process of charging a surface of an insulating substrate to atarget potential, the process comprising: providing the substrate withthe surface in a state at an approximate initial potential above thetarget potential; flooding the surface with charged particle such that acharge yield of scattered particles is less than one; reaching a steadystate at which the surface achieves the target potential. whereincoverage of the surface is achieved in a step-wise fashion.
 16. Theprocess of claim 12, wherein coverage of the surface is achieved usingswathing. 17-22. (canceled)
 23. An apparatus for charging a surface ofan insulating substrate to a target potential, the apparatus comprising:an electron flood gun including a cathode, an anode, and a lowerelectrode; a substrate holder having a substrate bias voltage appliedthereto; a controller for controlling the flood gun and the substratebias voltage, wherein the controller is configured to flood the surfaceof the substrate with a higher-energy electron beam such that anelectron yield is greater than one and to subsequently flood the surfacewith a lower-energy electron beam such that the electron yield is lessthan one.
 24. The apparatus of claim 23, wherein, during thehigher-energy flooding, the lower electrode is set at an initialpotential that is slightly higher than the target potential, and thesurface approximately reaches the initial potential.
 25. The apparatusof claim 24, wherein, during the lower-energy flooding, the cathode isset at the target potential, and the surface achieves the targetpotential.
 26. A process of charging a surface of an insulatingsubstrate to a target potential, the process comprising multiple mirrormode flooding steps, wherein each mirror mode flooding step includes:applying a voltage to the surface; and flooding the surface with chargedparticle such that a charge yield of scattered particles is less thanone.
 27. The process of claim 26, wherein decreasing voltages areapplied to the surface until the target potential is reached.